Process for the preparation of sintered zinc powder battery electrodes

ABSTRACT

Sintered zinc powder battery electrodes are prepared by (1) rinsing zinc powder with a solvent that dissolves zinc oxide at a faster rate than it dissolves zinc and which is saturated with a water soluble salt having a higher melting point than zinc; (2) pouring off the solvent; (3) mixing the zinc with the water soluble salt used in step (1); (4) packing the mixture around a metal screen that is made of a metal which is an electrical conductor having a higher melting point than zinc; (5) heating the packed article at a temperature above 419* C but below the melting point of the water soluble salt and the metal screen; (6) cooling the article; and, (7) soaking the article in water to remove the water soluble salt.

Unite tates Weller tet [4 1 May 16, 1972 [72] Inventor: Richard D.Weller, Laurel, Md.

[73] Assignee: The United States of America as represented by theSecretary of the Navy [22] Filed: June 24,1970

[21] Appl.No.: 49,548

[52] U.S.Cl ..l36/30, 136/126, 75/201 [51] Int. Cl. ..H0lm 41/00 [58]Field ofSearch ..136/31, 30, 125-130, 136/120; 75/201, 211, 214, 222,223, 224;}64/49, 104, 1 ll [5 6] References Cited UNITED STATES PATENTS3,322,535 5/1967 Rao ..75/201 3,337,336 8/1967 Rao ..75/201 3,384,4825/1968 Kelly et a1.... .....75/201 3,348,976 10/1967 Kelly et a1..136/30 2,561,943 7/1951 Moulton ..136/31 3,071,638 1/1963 Clark et a1.....136/3l 3,397,057 8/1968 Harrington et al ..75/21 1 1,996,220 4/1935Tigerschiold et a1. ..264/1 1 1 3,427,204 2/1969 Clune et a1 ..136/30Primary Examiner-Winston A. Douglas Assistant Examiner-C. F. LefevourAttorney-R. S. Sciascia, J. A. Cooke and R. J. Erickson [57] ABSTRACTSintered zinc powder battery electrodes are prepared by l rinsing zincpowder with a solvent that dissolves zinc oxide at a faster rate than itdissolves zinc and which is saturated with a water soluble salt having ahigher melting point than zinc; (2) pouring off the solvent; (3) mixingthe zinc with the water soluble salt used in step (1); (4) packing themixture around a metal screen that is made of a metal which is anelectrical conductor having a higher melting point than zinc; (5)heating the packed article at a temperature above 419 C but below themelting point of the water soluble salt and the metal screen; (6)cooling the article; and, (7) soaking the article in water to remove thewater soluble salt.

9 Claims, N0 Drawings PROCESS FOR THE PREPARATION OF SINTERED ZINCPOWDER BATTERY ELECTRODES BACKGROUND OF THE INVENTION This inventiongenerally relates to battery electrodes and more particularly tosintered zinc powder battery electrodes.

Zinc electrodes have been made in the past by simple compacting ofpowder or granular zinc. These electrodes, however, have not had truemetal to metal bonds because during sintering zinc oxide forms on thesurface of each individual particle preventing such contact. As aconsequence, these electrodes have poor mechanical strength and lose agood portion of the available zinc due to dropping off of particlesduring discharge and electrical isolation of portions of the electrodefrom the external circuit.

Other previously employed methods of producing zinc electrodes includeforming amalgams with mercury, forming composites which contain organicmaterials and wrapping loose zinc powder in a bag of porous material.Again, the electrodes produced by these prior art methods leave much tobe desired.

SUMMARY OF THE INVENTION Accordingly, one object of this invention is toprovide a process for the preparation of sintered zinc powder batteryelectrodes.

Another object of this invention is to provide sintered zinc powderbattery electrodes which have relatively good mechanical strength.

Still another object of this invention is to provide sintered zincpowder battery electrodes which have a relatively high efficiency ofzinc conversion.

These and other objects of this invention are accomplished by providinga process to produce sintered zinc powder battery electrodes comprising:(1 rinsing zinc powder with a solvent that dissolves zinc oxide at afaster rate than it dissolves zinc and which is saturated with a watersoluble salt that has a higher melting point than zinc; (2) pouring offthe solvent; (3) mixing the damp zinc with the water soluble salt usedin Step l (4) packing the mixture around a screen made of a metal whichis an electrical conductor and which has a higher melting point thanzinc; (5) heating the packed article at a temperature above 419 C butbelow the melting point of the water soluble salt or the metal screen;(6) cooling the article; and, (7) soaking the article in water to removethe water soluble salt.

DESCRIPTION OF THE PREFERRED EMBODIMENT The process of the instantinvention to produce sintered zinc powder battery electrodes comprises anumber of steps to be performed.

First, the zinc powder is rinsed with a solvent that can dissolve zincoxide at an appreciably faster rate then it dissolves zinc in order toremove all the zinc oxide that has formed on the powdered zinc. The zincpowder may have just about any particle size since particle size is nota critical feature of this invention, although a preferred size issmaller than 60 mesh. This solvent should be saturated with a watersoluble salt which has a higher melting point than zinc. The same saltin powdered form is also used in step (3), hereinafter described ingreater detail. Common solvents that can be used in this step of theinstant process include acids such as HCI, H 50, and HNO (preferablyabout IN or between 0.25N and about 1N), NaOH KOH and NH OH (5-40percent by weight solutions). Other solvents which dissolve zinc oxideat an appreciably faster rate than they dissolve zinc can be used. Thecontact time is preferably rather short, generally in the neighborhoodof about 30 to 60 seconds because some of the solvents, such as theacids will start to attack the zinc if contacting is continued for alonger period of time. When a solvent that does not attack zinc is usedthe contacting may continue indefinitely.

After contacting the solvent is poured ofi leaving the zinc powder in adamp condition. The damp zinc powder is then mixed and packed with apowdered water soluble salt that has a higher melting point than zinc.This is the salt which was used to saturate the solvent in the firststepof this process. The particle size of the salt, like that of the zinc,is not critical but is preferably a size smaller than 60 mesh. The ratioof the salt used to the zinc powder can be varied depending on thequality of the zinc electrode desired. Preferably the ratio of salt tozinc powder varies between 0.38 and 1.5. Some water soluble salts thatmay be used in this step are NaCl, KCl, LiCl etc. Any of the commonlyused metal screens are to be included in the middle of the electrode asthe mixture of salts is compacted. The metal screen should, of course,be electrically conductive and have a melting point that is not onlyhigher than that of zinc but higher than the temperature to which theelectrode will be heated in the heating step, hereinafter described.Commonly used screens are made of silver, nickel, copper and iron. Thepacking is carried out in any conventional manner.

After packing the electrode is heated, preferably in a furnace with aninert atmosphere. The electrode must be heated to at least the meltingpoint of zinc (419 C) but must not be heated to a temperature which willmelt either the water soluble salt or the metal screen inserted into theelectrode. After the zinc melts the electrode is held together by thesalt and the zinc is held in place by surface tension. All that isnecessary is that the temperature be kept above 419 C until the zinc hasmelted. As will be understood by one of ordinary skill in the art, thezinc in the packed article will melt when the furnace reaches 419 C butthe article is preferably kept in the furnace for a short period of timeafter it reaches 419 C or the temperature is raised beyond 419 C toinsure that all the zinc present in the electrode has melted. However,it should not be kept above 4l9 C for more than about 5 minutes to avoidpuddling of the zinc. With coarse size particles a heat treatment for ashorter period of time than with the finer size particles may be had.

After heating the electrode is allowed to cool and then soaked in waterin order to remove the water soluble salt. This leaves the desiredsintered zinc powder battery electrode with the metal screen therein.

The general nature of the invention having been set forth, the followingexample is presented as a specific illustration thereof. It will beunderstood that the invention is not limited to this specific examplebut is susceptible to various modifications that will be recognized byone of ordinary skill in the art.

EXAMPLE 20 grams of zinc powder was rinsed with 1N HCI which wassaturated with NaCl. Contacting was maintained for about 30 seconds inorder to remove the zinc oxide coating from the zinc particles. Most ofthe HCI was then poured off leaving the zinc powder damp. 8 grams ofNaCl powder was added to the wet zinc making a damp mixture of NaCl andzinc powers. This mixture was packed into a die while it was still dampand pressed to 7,000 psi into an electrode which was approximately 1% X1% X /s inches. A silver metal screen was included in the middle for acurrent collector. The pressed electrode, still damp, was held betweentwo discs of sintered glass and placed into an argon atmosphere furnacewhich was heated up to 500 C in approximately l5 minutes. Thistemperature was then held for about 1 more minute. After cooling theelectrode was removed and soaked in water to dissolve the NaCl thusleaving the zinc electrode.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. A process for the production of a zinc powder battery electrodecomprising a. rinsing zinc powder with a solvent selected from the groupconsisting of alkaline and acid solvents which dissolves zinc oxide at afaster rate than it dissolves zinc so as to remove zinc oxide and whichis saturated with an inorganic water soluble salt which has a highermelting point than zinc;

. pouring off said solvent to obtain clamp zinc powder;

. mixing said damp zinc powder with the powder of said inorganic watersoluble salt that has a higher melting point than zinc used in step (a);

. packing the mixture, obtained in step (c), around a metal screenwherein said metal screen is a conductor of electricity and is made of ametal that has a higher melting point than zinc;

heating the packed article of step (d) in an inert atmosphere at atemperature above 419 C but below the melting point of said metal screenand said inorganic water soluble salt for a time period not to exceed 5minutes;

. cooling said article, and

g. soaking said article in water to remove said inorganic water solublesalt.

2. The process according to claim 1 wherein the particle size of saidpowdered zinc and said powdered inorganic water soluble salt is smallerthan 60 mesh.

3. The process according to claim 2 wherein the ratio of the inorganicwater soluble salt to zinc is between 0.38 and l .5.

4. The process according to claim 3 wherein said inorganic water solublesalt is selected from the group consisting of NaCl, KC] and LiCl.

5. The process according to claim 4 wherein said solvent to dissolve thezinc oxide is selected from the group consisting of 5-40 percent NaOH,KOH and NH OH.

6. The process according to claim 4 wherein said solvent to dissolve thezinc oxide is selected from the group consisting of about 0.25N to aboutlN HCl, H 80, and HNO 7. A process according to claim 5 wherein saidmetal screen is composed of a metal selected from the group consistingof silver, nickel, copper and iron.

8. A process according to claim 6 wherein said metal screen is composedof a metal selected from the group consisting of silver, nickel, copperand iron.

9. A process according to claim 8 wherein said solvent to dissolve zincoxide is about 1N HCl, said water soluble salt is NaCl and said metalscreen is silver.

2. The process according to claim 1 wherein the particle size of saidpowdered zinc and said powdered inorganic water soluble salt is smallerthan 60 mesh.
 3. The process according to claim 2 wherein the ratio ofthe inorganic water soluble salt to zinc is between 0.38 and 1.5.
 4. Theprocess according to claim 3 wherein said inorganic water soluble saltis selected from the group consisting of NaCl, KCl and LiCl.
 5. Theprocess according to claim 4 wherein said solvent to dissolve the zincoxide is selected from the group consisting of 5-40 percent NaOH, KOHand NH4OH.
 6. The process according to claim 4 wherein said solvent todissolve the zinc oxide is selected from the group consisting of about0.25N to about 1N HCl, H2SO4 and HNO3.
 7. A process according to claim 5wherein said metal screen is composed of a metal selected from the groupconsisting of silver, nickel, copper and iron.
 8. A process according toclaim 6 wherein said metal screen is composed of a metal selected fromthe group consiSting of silver, nickel, copper and iron.
 9. A processaccording to claim 8 wherein said solvent to dissolve zinc oxide isabout 1N HCl, said water soluble salt is NaCl and said metal screen issilver.